BACKUP DATABASE [Databasename] TO DISK "[Where on the disk to place the backup file Both the Path and the .bkp file]"
ex.
BACKUP DATABASE TestDB TO DISK "C:\Program Files\Microsoft SQL Server\MSSQL\Backup\Testdb.bak"

Use this for a log backup

BACKUP LOG [Databasename] TO DISK "[Where on the disk to place the backup file Both the Path and the .bkp file]"
ex.
BACKUP LOG TestDB TO DISK "C:\Program Files\Microsoft SQL Server\MSSQL\Backup\Testdb.bak"